All higher organisms process sensory cues from the environment and adjust their behavior accordingly in order to survive. Understanding the neural circuits that underlie sensory processing is a fundamental aim of neuroscience, and will help in understanding the basis of many human diseases. Drosophila is particularly amenable to such study and recently developed genetic tools allow for precise manipulation of genes and neural circuits. To date, much of what is known of circadian behavior and visual system organization comes from work in Drosophila. This proposal utilizes a well-defined light avoidance assay in larvae to map the neural circuitry underlying visual behavior. Light avoidance requires function of the lateral neurons (LNs), which have been primarily studied for their role as neurons central to circadian rhythms. The simplicity and robustness of this behavioral assay, in combination with the diminished complexity of the larval brain makes this an ideal system for the mapping of neural circuits underlying behavior. Through genetic manipulation of visual neurons, LNs, and potential LN target neurons, this proposal seeks to determine the precise circuitry and neurotransmitters by which visual cues are transmitted to higher order brain structures. Understanding the mechanism by which LNs govern light avoidance should provide valuable insight into how neural circuits control behavior.